drivers: leds-qpnp-flash: Synchronize brightness_set and worker thread

Synchronize between the brightness_set and worker thread by using
led_access mutex lock from LED class device. This makes sure that
the brightness level does not get changed when the register write
is in progress and the last set brightness level always
gets applied.

CRs-Fixed: 1034167
Change-Id: Ic15810b6a7fa1b2f658ce8415cd4767df29c9daa
Signed-off-by: ansharma <ansharma@codeaurora.org>